Wells producing hydrocarbons are provided with pipes made of relatively thin pipes functioning as casings. Casings are formed by jointing a large number of pipe sections. Each pipe section is at one end provided with a male end having externally arranged threads, while the opposite end is provided with a female end having internally arranged threads, adapted to the threads on the male end, the casing being extended by screwing a pipe section with its male end into a female end of an adjacent pipe section. Such jointed pipe sections are run unexpanded down into a well. When the casing has reached it position cement may, if required, be run down through the unexpanded casing in order to fill the annulus between the hole of the well bore and the casing with cement. The unexpanded casing is then expanded by driving a cone body having a larger diameter than the diameter of the unexpanded casing down into casing. Such conical bodies are often a cone which may be made of a large steel element or made of several assembled smaller sector shaped elements. Alternatively, the cone may also comprise of cylindrical rollers having somewhat skewed or slanted axes with respect to the longitudinal axis of the pipe. It is also possible to apply hydraulic excess pressure in order to deform the pipe, where such excess pressure preferably may be used in combination with one of the conventional expansion methods.
With casings of this type there is a need for maintaining a threaded joint which is both structurally intact and also is gas tight preferably prior to, but in particular, subsequent to expansion, since such a casing often is positioned in a formation with an external or internal gas pressure, and since there always is a requirement of controlling the pressure inside the gas pipe.
In the expansion phase of the installation there is a problem that the threaded joint due to the radial and partly also axial expansion may loosen, since the threaded connection between the male end of one pipe section and the female end of an adjoining pipe section during this operation will be bent at least twice, unless the expansion also is based upon hydraulic expansion. The first bending occurs at the moment when the conical surface of the expansion tool hits that part of the pipe, then when the curving stretches out due to the cone movement along the joint, and then due to the curving the pipe is given when the cone is leaving and finally when the cone is pulled out.
Studies and simulations of the expansion indicate that in particular the internal start of a threaded joint of the casing and the corresponding external end of the threaded joint of the casing are exposed in particular to radial stresses, loads and movements as a consequence of the expansion.
When designing the end sections to be jointed with each other by means of a threaded joint, there is in particular a need for a joint which amongst others is suitable for remaining in locked and gas tight engagement even during the phase where the thin walled pipe is expanded and also subsequent to the expansion. Likewise there is a need for a pipe joint which may withstand cyclic loads and fatigue, tensional loads and compressive loads, and bending moments, and in certain instances also the rotational moment, without risking that the threaded joint unintentionally is loosened or weakened in any way, for example during running of such a pipe string down into a deviating well.
From US 2003/01937376 a threaded joint for pipe sections is known, jointed to form a pipe line for transport of hydrocarbons. The object of this solution is to be able to resist radial plastic expansion of the pipe line. Seen as a section in the longitudinal direction of the threaded parts, each thread flank is for this purpose provided with pairs of adjacent, skewed plane surfaces meeting in a single contact point, extending into the thread opening, whilst the thread crest and thread root is plane. In such way a locking effect against radial movement of a pipe with respect to the jointed pipe is achieved.
U.S. Pat. No. 4,004,832 describes a casing coupling having internally arranged threads to be used for jointing adjoining ends of two drill string sections. The drill string sections are at each end provided with a tapered end, provided with externally arranged threads configured to cooperate with the internally arranged threads of the coupling. The threads have thread crests and thread roots forming circular arcs and having a short flank, giving the threads an open shape.
U.S. Pat. No. 2,909,380 describes a corresponding thread shape where the thread crests and the thread roots are formed of circle arcs and where the intermediate flanks are inclined, providing an open thread shape.
NO 20083915, which belongs to the applicant and which is hereby included by the reference with respect to the use of deformable metal seals, discloses a gas tight pipe shaped coupling or joint used in connection with production of oil and/or gas, where the pipes are manufactured of tubular sections and where said sections, after being interconnected at their respective ends, are finally formed by expansion. The pipes are formed from at least two sections, one outer tubular section and one inner tubular section. The ends of each section are overlapping the ends of the next, succeeding section, whereby one or more of the inner. Intermediate or outer tubular sections are of different metallic materials and/or different thickness. Under the deformation process the sections are deformed plastically in the overlapping zone, forming a metallic seal and thereby providing gas pressure integrity between the inside and outside of the expanded pipe.